1. Field of the Invention
The present invention relates to a radiographic apparatus which can present fine contrast of image information.
2. Related Background Art
In general, a radiographic apparatus is used in the fields of medical radiography, industrial nondestructive radiography, and the like. The use mode of the apparatus will be described below with reference to FIG. 1. When radiation generated by a radiation source 1 is irradiated onto an object S, the radiation is intensity-modulated and scattered in accordance with the internal structure of the object S owing to interactions such as absorption, scattering, and the like of the object S with respect to the radiation, and then enters a radiographic unit 2. In the radiographic unit 2, a housing 3 which has a window portion for transmitting the radiation and intercepts light from its interior, a grid 4 for removing unwanted scattered radiation produced by the object S, a phosphor 5 for converting the radiation into fluorescence, and an image receiving means 6 sequentially are arranged.
The radiation reaches the grid 4 via the radiation window portion of the housing 3. The grid 4 is normally a plate obtained by cutting a multi-layered member obtained by alternately stacking lead plates and aluminum plates, and removes unwanted scattered radiation produced by the object S by matching the directions of the nearly parallel lead plates with the primary radiation traveling direction, thus improving the contrast of a radiographic image which is transmitted through the grid 4.
In general, as the phosphor 5, an intensifying screen obtained by applying CaWO.sub.4 or Gd.sub.2 O.sub.2 S:Tb on a support material, or a fluorescent crystal such as CsI is used. Since the phosphor 5 has characteristics of emitting fluorescence at an intensity proportional to the dose of radiation, the radiographic image is converted into a visible light image by the phosphor 5. The image receiving means 6 disposed behind the phosphor 5 generates an image corresponding to the received light amount, and the visible light image generated by the phosphor 5 is converted into an image corresponding to its light amount by the image receiving means 6.
Normally, the image receiving means 6 comprises a film, and the radiographic image is recorded as a latent image that gives a photographic density nearly proportional to the logarithm of the amount of fluorescence on the film. After development, the recorded image is presented as a visible image, which is used in diagnosis, inspection, and the like.
Also, a computed radiography (CR) apparatus using an imaging plate applied with a BaFBr:Eu phosphor and BaF:Eu phosphor as photostimulable phosphors is also used. When the imaging plate that has been primarily excited upon irradiation of radiation is subjected to secondary excitation using visible light such as a red laser beam or the like, emission called photostimulated fluorescence is produced. The CR apparatus detects this emission using a photosensor such as a photomultiplier or the like, thereby acquiring a radiographic image.
Furthermore, recently, a technique for acquiring a digital image using, as the image receiving means, a photoelectric conversion apparatus in which pixels each made up of a very small photoelectric conversion element, switching element, and the like, are arranged in a matrix, has been developed.
FIG. 2 is an explanatory view of a conventional radiographic apparatus using a photoelectric conversion apparatus. A photoelectric conversion apparatus 7 serving as a light-receiving means and consisting of amorphous silicon is disposed behind a phosphor 5. In the photoelectric conversion apparatus 7, a plurality of pixels are formed in a matrix by stacking various semiconductor layers on the surface, at the side of the phosphor, of a transparent glass substrate having a thickness of several mm and both surfaces polished.
A radiation source 1 is connected to the output from a radiation generating apparatus 11, and the output from a radiographic unit 2 is connected to an A/D converter 12. The radiation generating apparatus 11 and A/D converter 12 are connected to a CPU 14, temporary storing apparatus 15, external storing apparatus 16, and display apparatus 17 via a bus line 13.
In general radiography, radiation generated by the radiation source 1 in response to a signal from the CPU 14 is transmitted through, absorbed, and scattered by the object S.
This radiation is converted into fluorescence by the phosphor 5 via the grid 4, and is further converted into visible light by the phosphor 5. The converted visible light illuminates the pixels on the photoelectric conversion apparatus 7, which detects that light as a radiographic image analog signal having information of the object S. The radiographic image analog signal- is converted into a digital signal by the A/D converter 12, and the digital signal is transferred to the temporary storing apparatus 15. Also, the digital signal is transferred to and stored in the external storing apparatus 16. Furthermore, the digital signal is subjected to an image process suited for diagnosis, and is indicated on the display apparatus 17 for the purpose of diagnosis.
On the other hand, a radiographic apparatus in which a phosphor is stacked on two-dimensional photoelectric conversion elements comprising CCDs, amorphous silicon or amorphous selenium is known.
An example of merits expected using an apparatus such as the CR apparatus, photoelectric conversion apparatus, and the like, which can directly acquire digital data, is as follows. An image process is facilitated, and correction of improper photographing condition, image emphasis of the region of interest, and the like can be easily achieved. Using an image communication means such as a facsimile apparatus or the like, expert doctors in urban hospitals can make diagnosis for patients in remote places without any expert doctors. Furthermore, when image digital data are stored in magnetooptical disks or the like, the storage space can be greatly reduced as compared to a case wherein films are stored. Also, since previous images can be easily searched, a reference image can be indicated more easily than a case wherein films are to be searched.
However, in the above-mentioned conventional arts, the output of the radiographic apparatus is unstable. In a screen film system using a film as the image receiving means, as shown in FIG. 1, the output becomes unstable due to the use of films. Films have sensitivity differences depending on their manufacturing lots and management conditions, and a sensitivity difference as large as about 10% is often observed. The film temperature upon photographing largely influences the sensitivity, and films photographed irrespective of these film sensitivity differences suffer large variations in photographic density. Furthermore, when photographed films are developed, the temperature of the developing solution, the developing time, and the fatigue of the developing solution often considerably change the photographic density. As described above, in the screen film system that uses films with unstable sensitivities and photographic densities as the image receiving means, it is very hard to obtain constant photographic densities. The system requires of a radiographic engineer much labor such as checking of the film sensitivity, maintenance of an automatic developing machine, and the like to stabilize the photographic density.
Even the CR apparatus that uses the imaging plate as the image receiving means suffers a problem called fading. Fading is a phenomenon in which the radiographic image information accumulated on the imaging plate upon irradiation of the radiation decreases over time until it is read. As is known, if one hour has elapsed at 32.degree. C., the light emission amount decreases by about 20 to 40%. The imaging plate is normally sealed in a portable light-shielding member called a cassette, and is carried into a photographing site. In an imaging plate reading apparatus, since the time elapsed from photographing until reading is unknown, the output becomes unstable due to fading. Also, since the fading characteristics are influenced by temperature, even when the time elapsed from photographing until reading is known, it is difficult to stabilize the output.
In order to solve the above-mentioned problem, the CR apparatus has a function of outputting a constant photographic density by changing the read gain of the imaging plate irrespective of the fading characteristics and the dose of radiation by utilizing its feature that enables acquisition of a digital image. On the other hand, the read gain value is often used as an index for the dose of radiation. However, since this gain value simultaneously corrects both the fading characteristics and dose of radiation, there is no means for detecting if the dose of radiation is truly proper. For this reason, if the dose of radiation is determined with reference to the gain value and output image, the object, especially a patient, may be exposed to excessive radiation in the next photographing.
Also, the radiographic apparatus using the photoelectric conversion apparatus 7 as the image receiving means, as shown in FIG. 2, suffers an unstable output. This is because the characteristics of the photoelectric conversion device 7 may change over time. The photoelectric conversion apparatus 7 is normally made up of semiconductor elements obtained by lightly doping a substance into silicon single crystal or amorphous silicon, and the optical input/output characteristics of a semiconductor element change in accordance with temperature. Also, when currents are accumulatively supplied to the semiconductor element, the semiconductor element deteriorates at certain odds, and its optical input/output characteristics may change. These factors also lower the output stability.
In the photoelectric conversion apparatus 7, as a method of solving this problem, a method of stabilizing the output using the optical input/output characteristics of the photoelectric conversion apparatus 7 that can be obtained from black- and white-level signals may be used. The black-level signal is an output signal obtained when no light enters the photoelectric conversion apparatus 7, i.e., a dark output, and the white-level signal is an output signal obtained when the predetermined amount of light Is input to the photoelectric conversion apparatus 7. It is effective to acquire at least one of the black- and white-level signals as needed and to stabilize the output. Especially, when the optical input/output characteristics of the photoelectric conversion apparatus 7 are to be accurately obtained, it is indispensable to acquire both the black- and white-level signals.
As an example of the method of stabilizing the output using the optical input/output characteristics obtained from the black- and white-level signals, the following method may be used. An input image signal, black-level signal, and white-level signal for a predetermined amount of light are acquired in units of pixels. The black-level signal is subtracted from the image signal and white-level signal to obtain a second image signal and second white-level signal. The black-level signal represents an offset in units of pixels, and the process for subtracting the offset is generally called offset correction. The predetermined light amount is then divided by the second white-level signal to obtain a gain signal in units of pixels. Furthermore, the second image signal is multiplied by the gain signal to obtain an output image signal from which the optical input/output characteristics of the photoelectric conversion apparatus 7 are calibrated. This process is generally called gain correction.
Let d(x, y) be the black-level signal, w(x, y) be the white-level signal, i(x, y) be the input image signal, o(x, y) be the output image signal, k(x, y) be the predetermined light amount distribution, and g(x, y) be the gain signal. Then, formulas for performing the above-mentioned processes for a two-dimensional image are: EQU w'(x, y)=w(x, y)-d(x, y) EQU i'(x, y)=i(x, y)-d(x, y) EQU g(x, y)=k(x, y)/w'(x, y) EQU =k(x, y)/{w(x, y)-d(x, y)} EQU o(x, y)=i'(x, y).times.g(x, y) EQU =k(x, y).times.{i(x, y)-d(x, y)} EQU /{w(x, y)-d(x, y)}
However, when the photoelectric conversion apparatus 7 is used in the radiographic apparatus, since it is stored in the housing 3 that shields the apparatus 7 from external light, it is impossible to acquire any white-level signal by inputting light to the photoelectric conversion apparatus 7. In order to acquire the white-level signal in this state, radiation must be irradiated onto the radiographic apparatus to cause stimulated emission of the phosphor 5 stored in the housing 3. However, irradiation of the radiation onto the radiographic apparatus without any patient in each photographing applies a heavy load on the radiographic engineer, and the inspection time per patient is prolonged. For this reason, it becomes hard to photograph a large number of patients within a short period of time. When radiation is frequently irradiated to acquire the white-level signal, the service life of a tube that generates radiation is shortened.
As a method of solving such problem, during warm-up operation of the radiation tube which is ordinarily done as a routine work upon starting up the radiographic apparatus, radiation is irradiated onto the radiographic apparatus to acquire black- and white-level signals. With this method, the output of the radiographic apparatus using the photoelectric conversion apparatus is expected to be stable. However, as has already been described above, since the output from the photoelectric conversion apparatus varies as temperature changes, the output may vary due to changes in characteristics of the photoelectric conversion apparatus in image acquisition at the startup timing of the apparatus.
FIG. 3 is a graph in which the abscissa plots the temperature of the photoelectric conversion apparatus, and the ordinate plots the output relative to the output at a temperature of 25.degree. C. The solid curve in FIG. 3 indicates the output of the photoelectric conversion apparatus when a predetermined amount of light is input, and the dotted curve indicates the dark output of the photoelectric conversion apparatus when light is shielded. As can be seen from FIG. 3, the output of the photoelectric conversion apparatus upon inputting a predetermined amount of light gradually increases as the temperature rises, while the dark output of the photoelectric conversion apparatus upon shielding light rapidly increases as the temperature rises. Hence, when black- and white-level signals are acquired at the startup timing of the radiographic apparatus, and a radiographic image photographed after the radiographic apparatus was sufficiently warmed up is calibrated using these signals, erroneous calibration may be done. As a consequence, the output of the radiographic apparatus becomes unstable depending on the warm-up state of the apparatus.
As described above, conventional radiographic apparatuses suffer unstable outputs irrespective of the types of image receiving means. In order to stabilize the output, much labor such as checking of the film sensitivity, maintenance of an automatic developing machine, and the like is required.